Given the following why is $($c$)$ correct please explain the green answer to $c$

$($b$)$ Write a variant for the abstract syntax of this language.

type bop larr

I Add

I Sub

$|$ Mul

I Div

type expr larr

$|$ Num of number

$|$ Var of string

FuncExpr of string $$ expr

$|$ CallExpr of expr $$ expr

I BopExpr of expr $$ bop $$ expr

$($c$)$ Formally define the big step semantics of this language.

$\frac{\text{v}\text{i}\text{n}\text{Z}}{vv}M$

$\frac{?}{E+xE(x)}$ VAR

$\frac{?}{E|--x*tdarr(x*t,E)}$FuNC

$\frac{E_1|--t_1(x*t_3,E_2),E_1|--t_2{v}_1,E_2[x|v_1]|--t_3{v}_2}{E_1|--t_1,t_2{v}_2}$ CALL

$\frac{E|--t_1{v}_1,E|--t_1{v}_2}{E|--t_1+t_2{v}_1+v_2}$ADD

$\frac{E|---t_1{v}_1,E|---t_1{v}_2}{E|---t_1-t_2{v}_1-v_2}$SUB

$\frac{E|---t_1{v}_1,E|---t_1{v}_2}{E|--t_1**t_2{v}_1**v_2}\frac{?}{MU}L$

$\frac{E|---t_1{v}_1,E|--t_1{v}_2}{E|--\frac{t_1}{t_2}\frac{v_1}{v_2}}$ DIV